A cock egg is an egg laid without a yolk. These may also be called wind eggs, but the term cock egg comes specifically from the belief that these eggs were laid by roosters. This at some point became tangled up with the idea that these eggs could hatch into a cockatrice, although presumably at the point that you discover that the egg had no yolk hatching it was no longer a possibility.
These eggs are most often the result of a young hen just starting to lay, and do not generally hatch into cockatrices. They may also be produced by older hens, and in fact wind eggs are also produced by other types of birds and even reptiles. While they may be caused by various factors, it is often the case that a bit of reproductive tissue has broken off from the ovary, stimulating the oviduct to treat it as a yolk and produce an egg around it; in these cases there may be a small greyish nucleus apparent in the egg white.
In the 1300s a 'cock's egg' was used to mean the runt of the litter, later generalized to a spoiled child or a meek person. Some etymologists believe that this is the source of the term cockney (used to refer to city folk in the 1500s, and later used to refer specifically to residents of the East End of London), although this is disputed.
Brevity Quest 2016